This invention relates generally to radar systems and more particularly to range gate positioning systems used in such radar systems.
As is known in the art, radar systems, such as pulse Doppler radar systems, are used to determine the range and/or relative velocity (i.e., Doppler velocity) of an object. Radar pulses are transmitted at a rate referred to as the pulse repetition frequency (PRF). The time interval between successive pulses is referred to as the pulse repetition interval (PRI). During a predetermined time after pulse transmission, radar return signals are sampled, or range gated, by the radar signal. That is, based on the difference in time between pulse transmission and the time which the sample is taken, each one of the samples corresponds to a range, or distance, between the radar system and the object producing the sampled return. The process is referred to as range gating, where each time a sample is taken represents a range cell, or gate, of the return produced by the object at the range corresponding to the time at which the sample is taken.
In applications where there is a relative velocity (i.e., Doppler velocity) between the radar system and the object, in order to track the object, the time at which the radar return sample is taken after pulse transmission is changed in accordance with the relative velocity between the radar system and the object. Thus, if the object is moving away from the radar system, the time at which the radar return is sampled relative to the time the radar pulse is transmitted must increase from radar pulse to radar pulse at a rate proportional to the relative velocity, or Doppler velocity, between the radar system and the object. In like manner, if the object is moving towards from the radar system, the time at which the radar return is sampled relative to the time the radar pulse is transmitted must decrease from radar pulse to radar pulse at a rate proportional to the Doppler velocity between the radar system and the object.
In order to determine the Doppler velocity of the object, the radar returns from a plurality of transmitted radar pulses are processed. More particularly, each set of radar returns from a plurality of consecutively transmitted radar pulses is referred to as a dwell. The radar system produces a plurality of consecutive dwells. For each dwell, the radar system determines the average Doppler frequency of an object at one of a plurality of contiguous range gates. Fine Doppler velocity resolution generally requires the a large number of radar returns per dwell (i.e., a relatively large data collection period). For objects having relatively high Doppler velocities, this data collection period translates into a time period during which the object to radar system range can experience a large change. If the range accuracy is less than the object movement over the dwell, some type of dynamic range gate adjustment (i.e., range gate positioning system) is required in order to maintain the range to the object in the middle of each dwell and thereby enable the radar system to track the range to the object with maximum signal to noise ratio.
Several systems have been used range "walk" compensation or velocity aiding schemes to move range gates during a dwell to prevent the object from passing through the "window" (i.e., time duration) of the range gate. In general, however, these systems have three limitations: First, the range resolution (i.e., window time duration) to which the gate can be placed is typically limited to at least one clock period of the radar system timing generator (i.e., the radar system clock pulse period) and range resolution less than a full system clock pulse period may be required; or second, the system may utilize tapped delay lines having gate delays which may be shorter than the system clock pulse period but may not be precisely repeatable because of the analog nature of a delay line; or, third, the system uses multiple system clocks which are difficult to synchronize.